1. Field of the Invention
The present invention relates to a power supply control device for electrical loads used in an on-vehicle electronic control apparatus and, particularly, to a power supply control device which detects any breakage and shorting abnormality of a load power supply circuit for electrical loads connected to a vehicle body at a negative terminal thereof.
2. Description of the Related Art
There are various types of power supply control devices for on-vehicle electrical loads which control an on-off duty factor of a switching element connected between a driving power supply and an electrical load such that a target current to be passed agrees with a current detected by a current detection resistor. For example, such control devices include current control devices for linear solenoids which require a current constantly varying in a wide range and current control devices for fuel injection electromagnetic valves which are kept open by a constant low current after being opened rapidly.
Some of such current control devices employ an internal feedback control method in which a microprocessor generates a target current command and a switching drive command according to a deviation between the target current command a detected current, and others employ an external feedback control method in which a microprocessor only generates a target current command and in which a switching drive command is generated by a deviation integrating circuit provided outside the microprocessor in accordance with a deviation between the target command current and a detected current.
In either control method, a commutation diode is connected in parallel with a series circuit formed by a current detection resistor and an electrical load. When a switching element is turned off, the commutation diode returns a load current which has been passed through the electrical load until that time, whereby the load current is smoothed and the generation of a surge voltage is suppressed.
In any of the current control devices, a ground circuit inside the current control device and a DC driving power supply, which is an on-vehicle battery, are both connected to a vehicle both at negative terminals thereof. A negative terminal of an electrical load may be connected in various ways.
For example, in the case of a current detector for an inductive load disclosed in Patent Document 1 listed below, a switching element is connected to a positive terminal of a linear solenoid constituting an electrical load, and a negative terminal of the electrical load is first led into a current control device and is then connected to a vehicle body outside the current detector through a current detection resistor. An anode terminal of a commutation diode is connected to a ground circuit inside the current detector.
In the case of a linear solenoid fault detector disclosed in Patent Document 2 listed below, a switching element and a current detection resistor are connected to a positive terminal of a linear solenoid constituting an electrical load, and a negative terminal of the electrical load is connected to a vehicle body outside the fault detector. An anode terminal of a commutation diode is connected to a ground circuit inside the fault detector.
Patent Document 1: JP-2003-75476A, in particular, FIG. 1 and abstract
Patent Document 2: JP-2000-114039A, in particular, FIG. 1 and abstract
(01) Explanation of Technical Problems in the Prior Arts
In the case of the current detector disclosed in Patent Document 1, since the current detection resistor is connected to the negative terminal of the electrical load, a problem arises in that connectors are required for a positive pole side wiring for connecting the electrical load to the switching element and for a negative pole side wiring for connecting the load to the current detection resistor.
When there is a shorting fault of the electrical load or a ground fault that is interference or contact between the positive pole side wiring of the electrical load and the vehicle body, an over-current flows through the switching element because of a very small current limiting resistance incorporated in the switching element. Thus, even if the switching element is quickly turned off, a possibility of a temporary over-stress on the switching element still remains.
Further, when there is breakage of a wiring connecting the current detector connected to the negative terminal of the electrical load to the vehicle body or there is a contact failure of a connector, an excessive surge voltage is applied to the interior of the current detector, which can result in breakage of the current detector.
In the case of the fault detector disclosed in Patent Document 2, the number of wirings for the electrical load is reduced. In addition, even when a shorting fault of the electrical load or a ground fault of the positive pole side wiring occurs, the switching element can be safely protected by suppressing any over-current through the switching element with the current detection resistor and quickly turning the switching element off.
However, when the witching element opens, since the load current returned to the commutation diode returns through the ground circuit inside the fault detector, the ground potential in the fault detector can fluctuate to cause erroneous operations attributable to noises.
(02) Object of the Present Invention
It is an object of the invention to suggest a power supply control device for an on-vehicle electrical load which serves as simple measures to provide protection against a shorting fault of the electrical load or and a ground fault of a wiring on a positive pole side of the load and which is configured such that a load current will not be superimposed on an internal ground circuit of a power supply control unit.